An electroluminescent (EL) device is a self-light-emitting device which has advantages over other types of display devices in that it provides a wider viewing angle, a greater contrast ratio, and has a faster response time. An organic EL device was first developed by Eastman Kodak, by using small molecules which are aromatic diamines, and aluminum complexes as a material for forming a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
The most important factor to determine luminous efficiency in an organic EL device is a light-emitting material. Until now, fluorescent materials have been widely used as a light-emitting material. However, in view of electroluminescent mechanisms, phosphorescent materials theoretically show four (4) times higher luminous efficiency than fluorescent materials. Thus, recently, phosphorescent materials have been investigated. Iridium(III) complexes have been widely known as phosphorescent materials, including bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)indium (Ir(ppy)3) and bis(4,6-difluorophenylpyridinato-N,C2)picolinate iridium (Firpic) as red, green and blue materials, respectively.
In order to improve color purity, luminous efficiency and stability, light-emitting materials can be used as one prepared by mixing a dopant with a host material. In the host material/dopant system, the host material has a great influence on the efficiency and performance of an EL device, and thus is important.
At present, 4,4′-N,N′-dicarbazol-biphenyl (CBP) is the most widely known host material for phosphorescent substances. Further, Pioneer (Japan) et al. developed a high performance organic EL device employing, as a host material, bathocuproine (BCP) and aluminum(III)bis(2-methyl-8-quinolinate)(4-phenylphenolate) (BAlq) which had been a material used for a hole blocking layer.
Though these phosphorous host materials provide good light-emitting characteristics, they have the following disadvantages: (1) Due to their low glass transition temperature and poor thermal stability, their degradation may occur during a high-temperature deposition process in a vacuum. (2) The power efficiency of an organic EL device is given by [(π/voltage)×current efficiency], and the power efficiency is inversely proportional to the voltage, and thus the power efficiency should be high in order to reduce power consumption. Although an organic EL device comprising phosphorescent materials provides higher current efficiency (cd/A) than one comprising fluorescent materials, when the conventional materials such as BAlq or CBP are used as phosphorescent host materials, a significantly high driving voltage is necessary compared to an organic EL device using a fluorescent material. Thus, there is no merit in terms of power efficiency (Im/W). (3) Further, the operation lifetime of an organic EL device is short and luminous efficiency is still required to be improved.
International Patent Publication No. WO 2006/049013 discloses compounds for organic electroluminescent materials having a condensed bicyclic group as a backbone structure. However, it does not disclose a compound having a nitrogen-containing condensed bicyclic group, which is formed by condensing two 6-membered rings, a carbazolic group, and an aryl or heteroaryl group.